Far infrared interferometers in space would enable extraordinary measurements of the early universe, the formation of galaxies, stars, and planets, and would have great discovery potential. Since half the luminosity of the universe and 98% of the photons released since the Big Bang are now observable at far IR wavelengths (40 - 500 micrometers ), and the Earth's atmosphere prevents sensitive observations from the ground, this is one of the last unexplored frontiers of space astronomy. We present the engineering and technology requirements that stem from a set of compelling scientific goals and discuss possible configurations for two proposed NASA missions, the Space Infrared Interferometric Telescope and the Submillimeter Probe of the Evolution of Cosmic Structure.

The ASTRO-F project is currently in its final stage of proto-model, which is constructed same as flight-model. Since instrument goals of the Far-Infrared Surveyor (FIS) are unprecedented achievement of high sensitivity and high spatial resolution in far-infrared wavelength, the proto- model stage is important to prove the performance as the flight instrument. We mainly present here the latest optical, thermal, and mechanical properties of the proto- model of the FIS.

The design overview and current development status of the Infrared Camera (IRC) onboard the Japanese infrared space mission, ASTRO-F (commonly called as the Infrared Imaging Surveyor, IRIS), are presented. The IRC is one of the focal plane instruments of ASTRO-F and will make imaging and low- resolution spectroscopy observations in the wide spectral range of the near- to mid-infrared of 2 - 26 micrometers . ASTRO-F will be brought into an IRAS-type sun-synchronous polar orbit. The IRC will be operated in the pointing mode, in which the telescope will be pointed at a fixed target position on the sky for about 10 minutes. The pointed observation may be scheduled up to three times per orbit. The IRC has three channels: NIR (2 - 5 micrometers ), MIR-S (5 - 12 micrometers ) and MIR-L (12 - 26 micrometers ). All of the three channels use refractive optics. Each channel has a field-of-view of 10' X 10' with nearly diffraction-limited spatial resolution. The NIR and MIR-S channels simultaneously observe the same field on the sky, while the MIR-L observes the sky about 20' away from the NIR/MIR-S position. State- of-the-art large format array detectors manufactured by Raytheon/IRCoE are employed for the IRC. The NIR channel uses a 512 X 412 InSb array, and 256 X 256 Si:As IBC arrays are used for the MIR channels. Fabrication of the proto-model has been completed and the preliminary performance test is under way.

Planck associated to FIRST is one of the ESA scientific missions belonging to the Horizon 2000 program. It will be launched by an Ariane 5 in 2007. Planck aims at obtaining very accurate images of the Cosmic Microwave Background fluctuations, thanks to a spaceborne telescope featuring a wide wavelength range and an excellent control of straylight and thermal variations.

FIRST, the `Far InfraRed and Submillimeter Telescope', is the fourth cornerstone mission in the European Space Agency science program. It will perform photometry and spectroscopy in the far infrared and submillimeter part of the spectrum, covering approximately the 60 - 670 micrometers range.

The FIRST/Planck ESA program combines two ESA missions of the HORIZON 2000 program, the cornerstone mission of the Far InfraRed and Submillimeter Telescope and the third medium sized mission, Planck. An overview is given in this paper of the current system design, the performance parameters and an outlook on the spacecraft development.

FIRST (Far Infrared and Sub-millimeter Telescope) is one of the satellites of the next ESA scientific mission. FIRST/Planck, which will be launched in 2007 to the 2nd Lagrangian libration point L2. It will be a multi-user observatory, watching the universe in the infrared and sub- millimeter wavelength range from 60 to 670 micrometers . The paper will describe the latest design status of the cryostat, and its interfaces to the instruments and the telescope.

Composite materials are an ideal choice for the FIRST Telescope, since they provide dimensional stability, excellent stiffness to weight ratios, near zero thermal expansion, and manufacturing flexibility. The most challenging aspect of producing an all-composite FIRST telescope, is the development of the lightweight primary mirror. The design of the primary mirror must satisfy requirements for surface accuracy to operating temperatures of 80 +/- K as well as stiffness and strength considerations during launch.

SPIRE, the Spectral and Photometric Imaging Receiver, will be a bolometer instrument for ESA's FIRST satellite. Its main scientific goals are deep extragalactic and galactic imaging surveys and spectroscopy of star-forming regions in own and nearby galaxies. The SPIRE detectors are feedhorn- coupled NTD spider-web bolometers. The instrument comprises a three-band imaging photometer covering the 250 - 500 micrometers range, and an imaging Fourier Transform Spectrometer (FTS) covering 200 - 670 micrometers . The photometer has a field of view of 4 X 8 arcminutes which is observed simultaneously at 250, 350 and 500 micrometers with dichroic beam dividers separating the three spectral bands. Its angular resolution is determined by the telescope diffraction limit, with FWHM beam widths of approximately 17, 24 and 35 arcseconds at 250, 350 and 500 micrometers , respectively. An internal beam steering mirror can be used for spatial modulation of the telescope beam, and observations can also be made by scanning the telescope without chopping, providing better sensitivity for source confusion-limited deep surveys. The FTS has a field of view of 2.6 arcminutes and an adjustable spectral resolution of 0.04 - 2 cm-1 ((lambda) /(Delta) (lambda) equals 20 - 1000 at 250 micrometers ). It employs a dual-beam configuration with novel broad-band intensity beam dividers to provide high efficiency and separated output and input ports.

The SPIRE instrument for the FIRST mission will consist of a three band imaging submillimeter photometer and a two band imaging Fourier Transform Spectrometer (FTS) optimized for the 200 - 400 micrometers range, and with extended coverage out to 670 micrometers . The FTS will be used for follow-up spectroscopic studies of objects detected in photometric surveys by SPIRE and other facilities, and to perform medium resolving power (R approximately 500 at 250 micrometers ) imaging spectroscopy on galactic and nearby extra-galactic sources.

The Photoconductor Array Camera and Spectrometer is one of the three science instruments for ESA's Far Infra-Red and Submillimeter Telescope. It employs two 16 X 25 pixels Ge:Ga photoconductor arrays (stressed/unstressed) to perform imaging photometry and imaging line spectroscopy in the 60 - 210 micrometers wavelength band. In photometry mode, it will simultaneously image two bands, 60 - 90 or 90 - 130 micrometers and 130 - 210 micrometers , over fields of view of approximately 1' X 1.5' and approximately 2' X 3', respectively, with full beam sampling in each band. In spectroscopy mode, it will image a field of approximately 50' X 50', resolved into 5 X 5 pixels, with an instantaneous spectral coverage of approximately 1500 km/s and a spectral resolution of approximately 175 km/s. In both modes background-noise limited performance is expected, with sensitivities (5(sigma) in 1h) of 4 - 6 mJy or 2 - 8 X 10-18 W/cm2, respectively.

The Photoconductor Array Camera and Spectrometer (PACS) will be equipped with two sensor arrays consisting of 16 X 25 pixels each. Arranged in linear arrays of 16 detectors the sensitivity of the sensors is tuned to the wavelength ranges 60 micrometers to 130 micrometers and 130 micrometers to 210 micrometers , by applying different levels of stress to the Ge:Ga crystal utilizing a special leaf spring which is part of each of the 25 modules. The electronics of the sensors are mounted on the same module but thermally isolated from the sensor level which is at a lower temperature of about 2 K. The sensors are read out by a specially developed integrating and multiplexing cryogenic read-out electronics. With a fore optics made of light cones in front of the detector cavities a 100% filling factor is achieved and a high quantum efficiency close to 0.5 is expected. In order to achieve extremely good stress uniformity in all detectors and therefore equal cutoff wavelengths, a high degree of the quality of the Ge:Ga detectors and of the assembling components used for this dedicated stress mechanism is required. The first 6 engineering modules have been successfully manufactured and tested afterwards. The relative responsivity of a set of pixels has been determined and a good performance has been demonstrated for the sensors, which are very close to fulfilling the requirements for PACS aboard the infrared spectra telescope FIRST.

This paper describes a novel On-Board data compression concept of the FIRST/PACS mission of the European Space Agency. Using the lossy and lossless compression, the presented method offers a high compression rate with a minimal loss of potentially useful scientific data. It also provides higher signal to noise ratio than that for standard compression techniques. The various modules of the data compression concept are discussed in detail. We demonstrate the method on synthetic data.

In Bischof et al. (this conference) a novel on-board data reduction concept for FIRST/PACS is proposed, consisting of following modules: ramp fitting, integration, glitch detection and spatial/temporal redundancy reduction. In this paper we outline the experiments of the data reduction software on synthetic and astronomical data. These experiments demonstrate the feasibility of this novel approach. The evaluation of its core modules on observational data from ISO is presented. We mainly focus on the performances of the ramp fitting and the glitch detection modules.

The Heterodyne Instrument for FIRST is comprised of five SIS receiver channels covering 480 - 1250 GHz and two HEB receiver channels covering parts of 1410 - 1910 GHz and 2400 - 2700 GHz. Two local oscillator sub-bands derived from a common synthesizer will pump each receiver band. The synthesizer, control electronics and frequency distribution will be performed in the spacecraft service module. The service module will be connected in the local oscillator unit on the outside of the cryostat with a WR-28 waveguide for each of the 14 local oscillator sub-bands. the local oscillator unit will be passively cooled and thermally isolated from the cryostat wall. The module is comprised of seven units, one for each receiver band, containing two multiplier chains consisting of a k- to w-band multiplier, a MMIC power amplifier operating in one of five bands between 71 and 113 GHz, the high frequency multipliers, launching optics and electrical distribution. The entire assembly will be cooled to 120 K. The local oscillator system has the two field technical challenge of providing broad band frequency coverage at very high frequencies. This will be achieved through the use of high power GaAs MMIC amplifiers and planar diode multiplier technology in a passively cooled 120 Kelvin environment. The design criteria and the resulting overall system design will be presented along with a programmatic view of the development program and development progress.

The Heterodyne Instrument for the Far-Infrared and Sub- millimeter Telescope requires local oscillators well into the terahertz frequency range. The mechanism to realize the local oscillators will involve synthesizers, active multiplier chains (AMC's) with output frequencies from 71 - 112.5 GHz, power amplifiers to amplify the AMC signals, and chains of Schottky diode multipliers to achieve terahertz frequencies. We will present the latest state-of-the-art results on 70 - 115 GHz Monolithic Millimeter-wave Integrated Circuit power amplifier technology.

Several astrophysics and Earth observation space missions planned for the near future will require submillimeter-wave heterodyne radiometers for spectral line observations. One of these, the Far InfraRed and Submillimeter Telescope will perform high-sensitivity, high-resolution spectroscopy in the 400 to 2700 GHz range with a seven channel super- conducting heterodyne receiver complement. The local oscillators for all these channels will be constructed around state-of-the-art GaAs power amplifiers in the 71 to 115 GHz range, followed by planar Schottky diode multiplier chains. The Jet Propulsion Laboratory is responsible for developing the multiplier chains for the 1.2, 1.7, and 2.7 THz bands. This paper will focus on the designs and technologies being developed to enhance the current state- of-the-art, which is based on discrete planar or whisker contacted GaAs Schottky diode chips mounted in waveguide blocks. We are proposing a number of new planar integrated circuit and device topologies to implement multipliers at these high frequencies. Approaches include substrateless, framed and frameless GaAs membrane circuitry with single, and multiple planar integrated Schottky diodes. Circuits discussed include 200 and 400 GHz doublers, a 1.2 THz tripler and a 2.4 THz doubler. Progress to date, with the implications of this technology development for future Earth and space science instruments, is presented.

The Heterodyne Instrument for FIRST (HIFI) is a heterodyne receiver system which has an intermediate frequency (IF) amplifier that will likely exhibit 1/f-type gain fluctuations. Although the level of fluctuation is very small, wideband spectral observations require exceptional stability. A methodology for measuring 1/f fluctuations is described along with measurements of two amplifiers. Comparisons are made with previous 1/f measurements of HEMT amplifiers. The implications for HIFI are described.

We present a versatile digital autocorrelation spectrometer designed to suit the needs of HIFI, the sub-millimeter heterodyne instrument of the ESA's FIRST satellite. This spectrometer will offer a set of three observation modes with different `on-line resolution/total band-width' combinations (82 kHz/500 MHz, 163 kHz/1 GHz and 325 kHz/2 GHz). An original architecture based on mixed Gallium Arsenide and Silicon technologies, allowed us to realize a 1024 channel, low power consumption and high speed correlation module: 4 mW per channel at 550 MHz clock frequency. A prototype spectrometer has been developed. It includes a 2 X 250 MHz Image Rejection Mixer, a 500 MHz clock frequency analogue to digital converter, and two 1024 channel digital correlators. This model has been integrated and tested (in laboratory and on telescope). We expose these test results.

The wideband acousto-optical spectrometer (WBS) for HIFI- FIRST is comprised of two array-AOS with 4 times 1 GHz bands each. There are some advantages to this design, the most important one is that relative frequency and amplitude variations between the 4 bands are rather unlikely. This is demonstrated by laboratory tests, which verify also that fairly slow beam-switching at 0.5 Hz may be a sufficient chop speed for HIFI. The performance of array-AOS has also been demonstrated during measurements at ground-based observatories. WBS consists of three independent units, one IF-, one optics-, and one electronics-unit. Some of the details of the WBS design are described, and the present performance estimates are given.

An engineering model has been built for a space-borne 640- GHz SIS receiver. It is the key component of Superconducting Submillimeter-Wave Limb-Emission Sounder, which is to be operated aboard the Japanese Experiment Module of the International Space Station in 2005. The receiver includes two Superconductor-Insulator-Superconductor (SIS) mixers cooled at 4.5 K, as well as four High-Electron-Mobility- Transistor (HEMT) amplifiers, two of which cooled at 20 K and the other two at 100 K. These components are integrated in a compact cryostat with two-stage Stirling and Joule- Thomson refrigerators. The receiver components has been successfully cooled and the cryostat has survived random vibration tests. The 640-GHz SIS mixer, which uses a pair of Nb/AlOx/Nb junctions connected in parallel, is built so that a broad RF matching be achieved without mechanical tuners. It is followed by cooled low noise HEMT amplifiers with a noise temperature of less than 17 K. The total receiver noise temperature has been measured around 180 - 220 K over the bandwidth of 5.5 GHz.

Hybrid sensors performance critically depends on the performance of the analog read-out electronics. The analog design methodologies are very well known and documented provided the operating temperature stays above temperatures where freeze-out occurs. Even though the behavior of individual MOS transistors at low temperature, i.e. below 30 K, has been studied in detail, this has not yet led to design guidelines for the design of building blocks and or complete systems that will operate satisfactorily at these low temperatures. Here, we present some of these guidelines and their application to the design of a low power high gain amplifier.

A gallium arsenide photoconductive detector, which is sensitive in the far-infrared wavelength range from approximately 60 micrometers to 300 micrometers , offers the advantage of extending considerably the long wavelength cut-off of presently available photodetectors. FIRGA is an ESA sponsored GaAs detector development program which is approaching completion. The FIRGA study is intended to prepare the technology for large 2D GaAs detector arrays for far-infrared astronomy. The primary goal of the development is the preparation of a monolithic 32 element demonstrator array module with associated cryogenic read-out electronics. Continuous progress in material research has led to the production of pure and doped n-type GaAs layers using liquid phase epitaxy. We prepared sample detectors from those materials and investigated their electrical and infrared characteristics. Finally, a multi-layer structured detector device was manufactured. The 4 X 8 element array configuration is defined by sawing a split pattern into the layers with pixel size 1 mm X 1 mm. The device is back illuminated. The 32 pixels are connected to two cryogenic read-out electronics chips mounted close-by. Results of the initial detector performance tests are reported. We determined dark current, responsivity and response transients. Ongoing development activities will concentrate on material research, i.e. the production of n-GaAs layers of ultra-high purity and those with improved FIR characteristics using new centrifugal techniques for material growth.

The Photoconductor Array Camera and Spectrometer (PACS) is one of the scientific core instruments on board of the ESA Horizon 2000 Cornerstone Mission FIRST: The PACS instrument can operate as a dual-band imaging photometer or as an integral-field spectrometer. The scientific instrument, designed for remote measurements of astronomical far- infrared emissions, incorporates several temperature levels between 1.7 and 15 K in order to keep the self-emission of the instrument at a low level.

The SPIRE instrument covers the 200 - 670 micron spectral range with a three-band, 4' X 8' diffraction limited field of view photometer, and a dual-band, 2.6' diameter field of view imaging FTS. Optimization of the photometer optics has been given a high priority in the instrument design, allowing an all-reflecting configuration with seven mirrors in one plane. The design corrects for the large tilt of the telescope focal plane due to the off-axis position of the SPIRE field of view, and provides two pupil images (where a beam steering mirror and a cold stop are located) and two field images (where a pick-off mirror for the spectrometer and the final image are located). A large back- focal length allows for dichroic band separators and beam folding mirrors. The spectrometer is a Mach-Zehnder-type, dual channel FTS providing two input and two output ports. The output ports are physically separated from the input ports, and the second input port is fed from a black-body source providing compensation of the telescope background, required to minimize the effect of jitter noise. Powered mirrors are used within the interferometer arms to minimize beam diameters and to leave maximum space for the scan mechanism. The complementary output ports are filtered by band-pass filters to provide the two spectral channels required.

The deployable optical telescope, the second project of the Air Force Research Laboratory's Integrated Ground Demonstration Laboratory, will demonstrate critical integration technologies associated with the next generation of beam expanders for space-based laser systems and large apertures for tactical surveillance systems. AFRL's development will be carried in cooperation with the contractor community and have direct ties to the future program offices that will utilize the DOT technologies. A flow down of total wavefront error acceptable for future operational systems has been used to derive DOT experiment requirements. The sub-scale DOT will demonstrate the initial deployment of a segmented primary and secondary tower in a 1-g laboratory environment.

The Air Force Research Lab is proposing a DoD partnership with NASA on NEXUS; a deployable optics flight demonstrator scheduled to launch in 2004. NEXUS is designed to demonstrate technologies for the Next Generation Space Telescope, primarily the deployment and wave front control of a 2.8 meter optical telescope in space.

Next generation optical space telescopes with apertures > 10 m for imaging, lidar, communications and directed energy focusing will be unable to use conventional technologies which are impractical or too costly. Our resolution is to construct a telescope from a lightweight, membrane primary, which is holographically corrected for surface distortions, in situ. In order to design a practical space telescope, a scheme by which temporal variations in the mirror surface, caused by thermal and gravitational stresses must be found. We present evidence that a primary static hologram combined with a secondary adaptive optics system may be the least expensive and simplest approach.

The aperture of monolithic space telescope primary mirrors placed on orbit is limited to payload faring diameters, the largest being about 4-meters. This requires a novel stowage approach for monoliths larger than 4-meters. Very large aperture telescopes, 50 to 100-meter diameters, planned for deployment in the next 10 to 20 years will also require very large mirror segments in an effort to manage the phasing of the entire surface. The larger the mirror panels the fewer that will be required for such apertures. If the mirrors can be made thin enough to be deformed into a cylinder or undeformed but closely nested, enough surface area can be placed on orbit to facilitate large aperture telescope mirrors. 8-meter monolithic mirrors can be rolled into a 2.5-meter diameter cylinder with the secondary support structure stowed in the cylinder to maximize the payload faring volume. Hyper-thin mirrors can be closely nested in order to maximize volume as well. Presented is a design and engineering model of a 0.9-meter diameter hyper-thin, ultra- lightweight spherical composite mirror and methods, which led to the fabrication of the mirror.

Future space telescopes rely on advances in technology to enable fabrication of primary mirrors with orders of magnitude more area, yet similar mass as current mirrors. This requires a shift of paradigm from the concept of the mirror as a rigid, stable unit, to the idea of the mirror as a system that uses active control to maintain the figure of a flexible surface. We discuss issues for this new class of optics and present status on a 2-m prototype mirror for NGST.

Large space based telescopes such as the Next Generation Space Telescope (NGST) have motivated the study of large deployable primary mirror concepts. This paper will explore the rationale used to develop the trade space between rigid body adjustment of segmented mirrors and full primary mirror active figure control. This discussion covers the relative merits of the two fundamental approaches with regard to complexity and system performance, including performance at cryogenic operating temperatures such as envisioned for NGST. Some of the areas covered will be mirror segment size as it relates to complexity of control, ability to address radius of curvature adjustment, impact of different mirror substrate materials choices, and other system implications such as launch loads. An area of trade considered is the amount of control achieved at the primary mirror compared to augmented control using a deformable mirror at a subsequent pupil plane.

This paper is intended to address accuracy issues associated with hygrothermal stability of ultra-lightweight composite mirror structures. Hygrothermal stability of a mirror is ultimately defined as its optical performance when subjected to temperature or moisture variations. Stability is dictated by a combination of material behavior and geometric configuration. Ideally, an isotropic material could be used that is lightweight, has high stiffness, and has no response to temperature or moisture variances. This type of material would therefore be independent of geometry. Quasi-isotropic laminated CFRP composite materials offer most of these characteristics, but are transversely isotropic with near zero hygrothermal response in the plane of the laminate and a relatively high response through the thickness. Typically, mirrors made from laminated material consist of a thin curved shell supported by an array of ribs. Interference problems arise at the rib/shell interface resulting in a `print-through' effect by the ribs. Also, adhesive used to bond the ribs to the shell pull the shell causing additional `print-through'. Additional sources of instabilities result from material variances, processing, and assembly. These multiple sources of instabilities superimpose onto each other resulting in the structures overall hygrothermal stability.

The mass of the primary mirror has dominated the mass of larger aperture (> 1 m class) telescopes. Spaceborne telescopes have much to gain from a significant reduction in areal density. Areal density is often limited by the stiffness to weight ratio of the primary mirror. Two key factors drive this criteria: telescope structural characteristics (launch and deployment) and fabrication requirements. A new class of hybrid composite mirrors has been designed, prototyped, and fabricated to demonstrate the advantage of the high stiffness to weight ratio of carbon fiber composite materials and the superior optical fabrication for low expansion glasses. This hybrid mirror utilizes a unique `set and forget' fabrication technique. A thin meniscus of glass is mounted to a stiff composite support structure using composite flexure rods. The meniscus is lightweighted using waterjet pocket milling and is conventionally polished to a precise radius of curvature. This meniscus is then supported on the flexures and actuated to a precise figure. The flexures are fixed and the actuators are removed. The substrate is then ion figured to achieve the final figure. The areal density of this mirror is 10 kg/m2. Surface figure on a 0.25 m aperture prototype was demonstrated at better than (lambda) /4 (visible) prior to ion figuring. Two 0.6 m mirrors are under fabrication. The design of the mirror and results of the fabrication and testing will be discussed.

Challenges in high-resolution space telescopes have led to the desire to create large primary mirror apertures. One such telescope is the Next Generation Space Telescope (NGST, 8-m primary). In order to accommodate launch vehicles, the optical systems using these large apertures are being designed to accommodate extremely lightweight, deployable, segmented primary mirrors. The requirements for these segments include: meter-class diameter, areal densities of the order of 15 kg/m2, aspheric surface figure, near infrared and visible spectrum operation, diffraction limited surface figure, high stiffness, tight radius of curvature matching, and excellent thermal stability. Operating temperatures for various systems include ambient as well as cryogenic ranges. A unique ceramic, carbon fiber reinforced silicon carbide, developed by the Industrieanlagen- Betriebsgesellschaft mbH, has shown potential for use as a mirror substrate. This paper presents the deign and predicted performance of this mirror system in various applications. Also included are issues related to the fabrication of the Advanced Mirror System Demonstrator.

Large space optics technologies are developed for government support civilian and defense applications. Within a funding constrained environment, partnerships among members of the large space optics community serve to accelerate the pace of technology development and insertion of technology products into space operations. Although missions and operating requirements are quite different for the partners, NASA and DOD have teamed to address areas of common concern. One particularly successful partnership activity is aimed at significantly reducing areal density, cost and fabrication time for large optics. Other opportunities are being explored among the government partners.

Lightweight, deployable space optics has been identified as a key technology for future cost-effective, space-based systems. The United States Department of Defense has partnered with the National Aeronautical Space Administration to implement a space mirror technology development activity known as the Advanced Mirror System Demonstrator (AMSD). The AMSD objectives are to advance technology in the production of low-mass primary mirror systems, reduce mirror system cost and shorten mirror- manufacturing time. The AMSD program will offer substantial weight, cost and production rate improvements over Hubble Space Telescope mirror technology. A brief history of optical component development and a review of optical component state-of-the-art technology will be given, and the AMSD program will be reviewed.

This paper presents experimental results relating to the Air Force Research Laboratory Precision Deployable Optics System (PDOS) ground demonstration. The PDOS experiment represents a sub-scale experimental test-bed for the demonstration of science and technology related to a large-aperture deployable space-based telescope systems. A description of the experimental test-bed is included. A description of microdynamic phenomena, referred to as `events' or `microlurches', observed during the test phase of the ground demonstration is presented. The performance of a three input, three output, high bandwidth structural controller operating in the presence of these events is presented and compared to the performance of the uncontrolled system.

In this communication, we propose a novel method for estimating the aberrations of a space telescope from phase diversity data. The images recorded by such a telescope can be degraded by optical aberrations due to design, fabrication or misalignments. Phase diversity is a technique that allows the estimation of aberrations. The only estimator found in the relevant literature is based on a joint estimation of the aberrated phase and the observed object. By means of simulations, we study the behavior of this estimator. We propose a novel marginal estimator of the sole phase by Maximum Likelihood. It is obtained by integrating the observed object out of the problem; indeed, this object is a nuisance parameter in our problem. This reduces drastically the number of unknown and provides better asymptotic properties. This estimator is implemented and its properties are validated by simulation. Its performance is equal or even better than that of the joint estimator for the same computing cost.

We present a novel and fast method for utilizing wavefront information in closed-loop phase-diverse image data. We form a 2D object-independent error function using the images at different focus positions together with OTFs of the diffraction limited system. Each coefficient in an expansion of the wavefront is estimated quickly and independently by calculating the inner produce of a corresponding predictor function and the error function. This operation is easy to parallelize. The main computational burden is in pre- processing, when the predictors are formed. This makes this method fast and therefore attractive for closed loop operation. Calculating the predictors involves error function derivatives with respect to the wavefront parameters, statistics of the parameters, noise levels and other known characteristics of the optical system. The predictors are optimized so that the RMS error in the wavefront parameters is minimized rather than consistency between estimated quantities with image data. We present simulation results that are relevant to the phasing of segmented mirrors in a space telescope, such as the NGST.

We have developed a focus-diverse phase retrieval algorithm to measure and correct wavefront errors in segmented telescopes, such as the Next Generation Space Telescope. These algorithms incorporate new phase unwrapping techniques imbedded in the phase retrieval algorithms to measure aberrations larger than one wave. Through control of a deformable mirror and other actuators, these aberrations are successfully removed from the system to make the system diffraction limited. Results exceed requirements for the Wavefront Control Testbed. An overview of these techniques and performance results on the Wavefront Control Testbed are presented.

Control algorithms developed for coarse phasing the segmented mirrors of the Next Generation Space Telescope (NGST) are being tested in realistic modeling and on the NGST wavefront control testbed, also known as DCATT. A dispersed fringe sensor (DFS) is used to detect piston errors between mirror segments during the initial coarse phasing. Both experiments and modeling have shown that the DFS provides an accurate measurement of piston errors over a range from just under a millimeter to well under a micron.

To eliminate the mounting stress of three flat reflective mirrors in Balloon-borne Solar Telescope, these optics were attached to their mounts with a thin adhesive layer. This method will also be used in Space Solar Telescope. The primary mirror will be attached to its mount with a thin adhesive layer after first procedure of polishing, then polished to its ultimate demand. In this paper, the theory of adhesive layer analysis is given, and FE model of mirror and its mount is established to analyze the behavior of adhesive layer and the mirror stress. Some experiments are taken to measure Young's modulus of adhesive layer. The possibility of this method in ground-based telescope is also analyzed.

Wavefront sensing in monochromatic light is insensitive to segment piston errors that are a whole number of waves. If the wavefront sensing is performed in several wavelengths, this ambiguity can be resolved. We give an algorithm for finding the correct phase, given multiple measurements in different wavelengths. Using this algorithm, the capture range of a wavefront sensor can be extended from on the order of +/- (lambda) /2 in piston to several waves. This relaxes the demands on an initial, coarse alignment method. The extended capture range depends on the selection of wavelengths available for phase measurements and the expected accuracy of the wavefront sensing method used.

A telescope simulator was built as part of the Nexus wavefront control testbed, an NGST technology experiment at NASA's Goddard Space Flight Center. This testbed was designed to demonstrate complete control of a segmented telescope, from initial capture of light, through coarse alignment and phasing, to fine phasing and wavefront control. The existing telescope simulator allows testing of the fine phasing and wavefront control steps. A small deformable mirror in the simulator allows generation of an unobscured aberrated wavefront, for use in exploring the range of measurement and correction using the testbed's image-based wavefront sensor and larger deformable mirror. An alternate path under development for the simulator will create a segmented wavefront using three spherical mirrors; three-degree-of-freedom mounts under each mirror enable alignment and phasing experiments that will cover most of the operation sequence. Details of the hardware design and performance will be presented.

The Far Ultraviolet Spectroscopic Explorer (FUSE) satellite was launched into orbit on June 24, 1999. FUSE is now making high resolution ((lambda) /(Delta) (lambda) equals 20,000 - 25,000) observations of solar system, galactic, and extragalactic targets in the far ultraviolet wavelength region (905 - 1187 angstroms). Its high effective area, low background, and planned three year life allow observations of objects which have been too faint for previous high resolution instruments in this wavelength range. In this paper, we describe the on- orbit performance of the FUSE satellite during its first nine months of operation, including measurements of sensitivity and resolution.

The Advanced Camera for Surveys (ACS) is a third generation instrument for the Hubble Space Telescope (HST). It is currently planned for installation in HST during the fourth servicing mission in Summer 2001. The ACS will have three cameras.

The Cosmic Origins Spacecraft (COS) is a new instrument for the Hubble Space Telescope that will be installed during servicing mission 4, currently scheduled for July 2003. The primary science objectives of the mission are the study of the origins of large scale structure in the universe, the formation, and evolution of galaxies, the origin of stellar and planetary systems and the cold interstellar medium. As such, COS has been designed for the highest possible sensitivity on point sources, while maintaining moderate ((lambda) /(Delta) (lambda) equals 20,000) spectral resolution. in this paper, the instrument design and predicted performance is summarized, as well as summary of the instrument flight and prototype component performance to date.

The Cosmic Origins Spacecraft (COS) will be the most sensitive UV spectrograph to be flown aboard the Hubble Space Telescope. The COS FUV and NUV channels will provide high sensitivity at resolution greater than 20000 over wavelengths ranging from 115 nm to 320 nm. We present a brief review of the instrument design and grating test plan as well optical test results for the first FUV grating delivered.

In June 1997, NASA made the decision to extend the end of the Hubble Space Telescope (HST) mission from 2005 until 2010. As a result, the age of the instruments on board the HST became a consideration. After careful study, NASA decided to ensure the imaging capabilities of the HST by replacing the Wide Field Planetary Camera 2 with a low-cost facility instrument, the Wide Field Camera 3. This paper provides an overview of the scientific goals and capabilities of the instrument.

The Space Sciences of Astronomy, Astro-Physics, and Astro- Biology could be advanced by ten years, perhaps more, if a faster, cheaper, better way than an entirely new spacecraft could be found to implement an 8-meter class observatory in space. Why 8 meters? Recent science results such as the Hubble Deep Field and other observations from the very large ground-based observatories suggest that to achieve the two prominent Space-Science goals of establishing the era of initial galaxy formation, and imaging and spectroscopy of Earth-like planets requires at least two magnitudes deeper imaging and a factor of six better resolution than anything now in existence or planned for UV/Optical wavelengths. The UVOWG Final Report lists agonizing details of critical science objectives toward these goals, agonizing because we cannot achieve them from the ground even with the four 8- meter mirrors of the VLTI. An 8-meter class space telescope will provide about 2.5 magnitudes deeper imaging and a factor of 6.5 better spatial resolution than the best we have now, HST. This paper describes a feasibility study for augmenting HST with 8-meter class optics. The results are very interesting, and surprising.